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The border region with Russia is very porous, unguarded in many places

Some contend that larger weapons have come into Ukraine from Russia

Under a blazing sun in early June, a group of pro-Russian rebels in eastern Ukraine were digging amid pine woods near the town of Krazny Liman.

Their grizzled commander was a bearded man in his 50s who would not tell us where he was from, but acknowledged that he wasn't local. He was proud to show off his unit's most prized possession -- a truck-mounted anti-aircraft unit that was Russian-made.

He told us the weapon had been seized from a Ukrainian base.

A few miles away, in the town of Kramatorsk, rebel fighters displayed two combat engineering tanks they said they had seized them from a local factory. Eastern Ukraine has long been a center of weapons production. They had parked one of the tanks next to the town square.

These were just two instances of how the rebels in eastern Ukraine were steadily adding more sophisticated weapons to their armory, including tanks, multiple rocket launchers -- and anti-aircraft systems.

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Who fired the Buk missile launcher?

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Video appears to show black box from MH17

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Video appears to show black box from MH1701:03

Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Debris from Malaysia Airlines Flight 17 sits in a field at the crash site in Hrabove, Ukraine, on September 9, 2014. The Boeing 777 was shot down July 17, 2014, over Ukrainian territory controlled by pro-Russian separatists. All 298 people on board were killed. In an October 2015 report, Dutch investigators found the flight was shot down by a warhead that fit a Buk rocket, referring to Russian technology, Dutch Safety Board Chairman Tjibbe Joustra said.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Australian and Dutch experts examine the area of the crash on August 3, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

A woman walks with her bicycle near the crash site on August 2, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Police secure a refrigerated train loaded with bodies of passengers from Malaysia Airlines Flight 17 as it arrives in a Kharkiv, Ukraine, factory on July 22, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

A pro-Russian rebel passes wreckage from the crashed jet near Hrabove on Monday, July 21, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

– Wreckage from the jet lies in grass near Hrabove on July 21, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

A man covers his face with a rag as members of the Organization for Security and Co-operation in Europe and the Dutch National Forensic Investigations Team inspect bodies in a refrigerated train near the crash site in eastern Ukraine on July 21, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Emergency workers carry a victim's body in a bag at the crash site on July 21, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

A piece of the plane lies in the grass in eastern Ukraine's Donetsk region on July 21, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

An armed pro-Russian rebel stands guard next to a refrigerated train loaded with bodies in Torez, Ukraine, on Sunday, July 20, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Ukrainian State Emergency Service employees sort through debris on July 20, 2014, as they work to locate the deceased.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

A woman covers her mouth with a piece of fabric July 20, 2014, to ward off smells from railway cars that reportedly contained passengers' bodies.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Toys and flowers sit on the charred fuselage of the jet as a memorial on July 20, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

People search a wheat field for remains in the area of the crash site on July 20, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

A woman walks among charred debris at the crash site on July 20, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Emergency workers load the body of a victim onto a truck at the crash site on Saturday, July 19, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Emergency workers carry the body of a victim at the crash site on July 19, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

A large piece of the main cabin is under guard at the crash site on July 19, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Victims' bodies are placed by the side of the road on July 19, 2014, as recovery efforts continue at the crash site. International officials lament the lack of a secured perimeter.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

A man looks through the debris at the crash site on July 19, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

An envelope bearing the Malaysia Airlines logo is seen at the crash site on July 19, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Armed rebels walk past large pieces of the Boeing 777 on July 19, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Ukrainian rescue workers walk through a wheat field with a stretcher as they collect the bodies of victims on July 19, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

A woman looks at wreckage on July 19, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Pro-Russian rebels stand guard as the Organization for Security and Co-operation in Europe delegation arrives at the crash site on Friday, July 18, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

A woman walks through the debris field on July 18, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Pro-Russian rebels stand guard at the crash site.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Wreckage from Flight 17 lies in a field in Shaktarsk, Ukraine, on July 18, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

A man covers a body with a plastic sheet near the crash site on July 18, 2014. The passengers and crew hailed from all over the world, including Australia, Indonesia, Malaysia, Germany and Canada.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

A diver searches for the jet's flight data recorders on July 18, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Coal miners search the crash site.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Wreckage from the Boeing 777 lies on the ground July 18, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

People search for bodies of passengers on July 18, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

A woman walks past a body covered with a plastic sheet near the crash site July 18, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Belongings of passengers lie in the grass on July 18, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

People inspect the crash site on Thursday, July 17, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

People walk amid the debris at the site of the crash.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Debris smoulders in a field near the Russian border.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Fire engines arrive at the crash site.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

A man stands next to wreckage.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Debris from the crashed jet lies in a field in Ukraine.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Family members of those aboard Flight 17 leave Schiphol Airport near Amsterdam, Netherlands.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

A large piece of the plane lies on the ground.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Luggage from the flight sits in a field at the crash site.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

A couple walks to the location at Schiphol Airport where more information would be given regarding the flight.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Flight arrivals are listed at the Kuala Lumpur International Airport in Sepang, Malaysia.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Debris from the Boeing 777, pictured on July 17, 2014.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

A man inspects debris from the plane.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

Wreckage from the plane is seen on July 17, 2014.

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A man talks with security at Schiphol Airport on July 17, 2014.

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Wreckage burns in Ukraine.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

A man stands next to the wreckage of the airliner.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

People inspect a piece of wreckage believed to be from Malaysia Airlines Flight 17. This image was posted to Twitter.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

People inspect a piece of wreckage believed to be from Malaysia Airlines Flight 17. This image was posted to Twitter.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

A piece of wreckage believed to be from Malaysia Airlines Flight 17. This image was posted to Twitter.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

A piece of wreckage believed to be from MH17. This image was posted to Twitter.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

An airsickness bag believed to be from MH17. This image was posted to Twitter.

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Photos:Malaysia Airlines Flight 17 crashes in Ukraine

A piece of wreckage believed to be from MH17. This image was posted to Twitter.

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In early June, they began to target Ukrainian planes and helicopters, with some success.

The day after we met the commander in the pine woods, an Antonov AN-26 transport plane was brought down over nearby Slovyansk.

Several Mi-8 and Mi-24 helicopters were also hit in this period, as was an Ilyushin IL-76 cargo plane near Luhansk -- it is about the size of a passenger jet.

Forty-nine military personnel were killed when the IL-76 crashed short of the airport.

For the most part, these aircraft were flying at relatively low altitudes, and were targeted by shoulder-launched SA-7 missiles and anti-aircraft guns. The pro-Russian rebels had taken control of several Ukrainian military depots and bases and stripped them of their weapons.

The SA-7 was standard Soviet issue. Relatively easy to operate, it is effective to altitudes of some 2,500 meters (8,000 feet).

But it and ZU 23-2 anti-aircraft batteries, which rebel units also obtained, are a world away from the SA-11 or "Buk" system that seems increasingly likely to have been used to shoot down Flight MH17 on Thursday.

Stealing a Buk

Could the pro-Russian rebels have acquired a serviceable Buk from a Ukrainian base and operated it? The evidence is circumstantial; a great deal of Ukrainian military hardware is in poor condition or redundant.

But on June 29, rebels raided the Ukrainian army's A-1402 missile facility near Donetsk. Photographs show them examining what they found.

The Russian website Vosti ran an article the same day titled "Skies of Donetsk will be defended by surface-to-air missile system Buk."

The article claimed: "The anti-air defense point is one of the divisions of the missile corps and is equipped with motorized "Buk" anti-aircraft missile systems."

Peter Felstead, an expert on former Soviet military hardware at Janes IHS, says that "the Buk is in both the Russian and Ukrainian inventories, but it's unclear whether the one suspected in the shoot-down was taken by rebels when they overran a Ukrainian base, or was supplied by Russia."

He told CNN that the Buk "would normally operate with a separate radar that picks up the overall air picture. This was almost certainly not the case with MH17," making it more difficult to identify the target and track its course.

Among the pro-Russian rebels are fighters who served in the Russian army. It is possible that some were familiar with the Buk, but Felstead agrees with the U.S. and Ukrainian assessment that Russian expertise would have been needed to operate it.

Photos:MH17: What they left behind

Photos:MH17: What they left behind

MH17: What they left behind – A birthday card found in a sunflower field near the crash site of Malaysia Airlines Flight 17 in eastern Ukraine, on Thursday, July 24. The passenger plane was shot down July 17 above Ukraine. All 298 people aboard were killed, and much of what they left behind was scattered in a vast field of debris.

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Photos:MH17: What they left behind

MH17: What they left behind – A classical music record is seen among the sunflowers on July 24.

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MH17: What they left behind – A shoe, appearing to be brand new, sits under foliage at the crash site.

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MH17: What they left behind – Two Dutch passports belonging to passengers lie in a field at the site of the crash on Tuesday, July 22.

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MH17: What they left behind – Clothing, sunglasses and chocolate are seen on July 22.

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Photos:MH17: What they left behind

MH17: What they left behind – More sunglasses and a travel guide lie in the field on July 22.

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Photos:MH17: What they left behind

MH17: What they left behind – A doll is seen on the ground on Saturday, July 19.

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Photos:MH17: What they left behind

MH17: What they left behind – A single shoe is seen among the debris and wreckage on July 19. There has been concern that the site has not been sealed off properly and that vital evidence is being tampered with.

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MH17: What they left behind – Pieces of a wristwatch lie on a plastic cover at the crash site.

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MH17: What they left behind – A toy monkey.

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Photos:MH17: What they left behind

MH17: What they left behind – Books, bags, a tourist T-shirt. Ukraine's government said it had received reports of looting, and it urged relatives to cancel the victims' credit cards. But a CNN crew at the scene July 19 said it did not see any signs of looting.

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Photos:MH17: What they left behind

MH17: What they left behind – Passports were scattered across the large field.

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MH17: What they left behind – Playing cards and euros are seen at the crash site.

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Photos:MH17: What they left behind

MH17: What they left behind – A travel guide and toiletries.

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MH17: What they left behind – Luggage on Friday, July 18.

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MH17: What they left behind – An empty suitcase is cordoned off near the plane's impact site on Thursday, July 17.

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"The system needs a crew of about four who know what they're doing. To operate the Buk correctly, Russian assistance would have been required unless the rebel operators were defected air defense operators - which is unlikely."

It is now the "working theory" in the U.S. intelligence community that the Russian military supplied a Buk surface-to-air missile system to the rebels, a senior US defense official told CNN Friday.

Russia has denied that any equipment in service with the Russian armed forces has crossed the border into Ukraine. And Aleksander Borodai, the self-described prime minister of the Donetsk People's Republic, said Saturday his forces did not have weapons capable of striking an aircraft at such a high altitude.

But someone in the border region where eastern Ukraine meets Russia has been using an advanced anti-air missile system.

Late Wednesday, the day before MH17 was presumably hit, a Ukrainian air force Sukhoi Su-25 combat jet was shot down close to the border with Russia.

The Ukrainian Defense Ministry told CNN that the plane was flying at 6,200-6,500 meters (about 21,000 feet) and was hit near a town called Amvrosiivka, which is only some 30 kilometers (20 miles) from where MH17 was hit and 15 kilometers (10 miles) from the border with Russia.

The Ukrainian military alleged the missile had been fired from Russian territory. It was the first time that a combat jet flying at high speed had been hit and came two days after an AN-26 -- flying at a similar altitude in the same area -- was shot down further north, in the Luhansk area.

Smuggling on the black roads

The Russian Defense Ministry said Friday that weapons could not be smuggled across the border "secretly." But they can.

By early June, rebels controlled several crossings along a stretch of border more than 200 kilometers (125 miles) long. The border area is open farmland that was neither patrolled regularly nor even marked in many places.

Dozens of unmonitored tracks known as black roads -- because they have been used for smuggling -- cross the border. Additionally, the Ukrainian border guard service was in disarray after an attack on its command center in Luhansk early in June.

On the road east toward the border through the town of Antratsyt there was no sign of a Ukrainian military or police presence. The pro-Russian rebels had already begun to bring across heavy weapons at that point.

A CNN team visited the border post at Marynivka in June, soon after a five-hour firefight involving border guards and members of the self-declared Vostok battalion of rebels who had been trying to bring over two Russian armored personnel carriers.

They had been abandoned during the battle.

The unknowns are these: Just how much weaponry has been brought in from Russia, how was it obtained, and did it include the SA-11 Buk?

In June, the U.S. State Department claimed that three T-64 tanks, several rocket launchers and other military vehicles had crossed the Russian border. Ukraine made similar accusations, saying the weapons had gone to Snezhnoe, a rebel stronghold close to where MH17 came down.

Even so, some experts, such as Mark Galeotti at New York University's Center for Global Affairs, say the evidence is largely circumstantial. NATO's images did not show the tanks actually crossing into Ukraine.

Wherever they came from, Russian language websites soon featured calls for people with military skills to call a number associated with the separatist Donetsk People's Republic if they could help operate or maintain the tanks.

One answered, "I served in the military engineering academy...and am a former commander in the intelligence."

But the separatists' greatest vulnerability was always from the air.

The Ukrainians had already shown, in driving them away from the Donetsk airport at the end of May, that they could use airpower to devastating effect. And they had begun to fly at higher altitudes to avoid shoulder-launched missiles.

To hold what remained of their territory, the pro-Russian rebels needed to be able to challenge Ukrainian dominance of the skies.

Whether they received help from across the border to do so, and in what way, is the question that governments around the world want answered.